1. Field of the Invention
The present invention relates generally to random access memories and more specifically, it relates to word line stitching in high-density dynamic random access memories.
2. Discussion of the Related Art
Today's highly advanced and sophisticated electronic systems increasingly require high-density and high-speed memories. The currently available technologies enable the development of high-density memories. While, the currently available process technologies provide the capability of designing and developing high-density memories, they also create stringent requirements that must be followed in design and layout of the devices.
Another factor that influences the design of the high-density memory devices is the cost of manufacturing. While, the market demands high-density memories, it also demands lower price for them. One way to reduce the cost is to design the high-density memory in the same die size used for a lower density memory. This means that memory cells are closer together and that the word lines and bit lines are manufactured much closer than before.
As the number of memory cells in a memory increases, so does the number of rows and columns of memory cells forming the array portion of the memory. An increase in the number of rows and columns while maintaining the total physical size of the array constant results in having the rows and columns, and thus the word lines and the bit lines, closer to each other.
One area in which the narrowing of the distance between the rows causes problem is in the area of word line stitching. Word line stitching is widely used in the design of memory devices. The word line is typically formed using poly layer, which has a substantially large equivalent resistance. To reduce the resistance of the word line, a metal layer is formed over the word line and is stitched to the word line in several intervals.
Referring now to FIG. 1, a typical word line stitching used in prior art is shown. It is noted that FIG. 1 is a fanciful depiction of how the word lines and metals are stitched together in the prior art. In addition, the dimensions shown in FIG. 1 are larger than the actual sizes for illustration purposes. FIG. 1 includes three word lines 10-14, three metal layers 16-20, and a plurality of contacts 22, contacts 24, and contacts 26. Typically, the word lines are fabricated from poly layers with metal layers being fabricated on the top of the word lines and are in parallel with them. Metal layer 16 is associated with word line 10, metal layer 18 is associated with word line 12, and metal layer 20 is associated with word line 14. Contacts 22 are used to stitch metal layer 16 to word line 10, contacts 24 are used to stitch metal layer 18 to word line 12, and contacts 26 are used to stitch metal 20 to word line 14.
Each contact 22 is associated with a poly contact overlap area 28 and a metal overlap area 30. Similarly, each contact 24 is associated with a poly contact overlap area 32 and a metal overlap area 34 and each contact 26 is associated with a poly contact overlap area 36 and a metal overlap area 38. Both the poly and metal contact overlaps are provided to assure a proper contact between the metal layers and the corresponding word line. As it can further be seen in FIG. 1, the contacts, including their respective overlap sections, are fabricated such that they aligned in a column.
It is very important from processing point of view that a minimum distance between the contacts as represented by "A" be maintained. Otherwise, the contact overlap areas could merge and short one word line to the adjacent word line. Furthermore, it is very important that the distance represented by B be less than or equal to one cell pitch. The cell pitch is defined as the total size of the memory cell. In the high-density memory devices having a much more compact architecture, it is very hard to maintain the minimum required distances using the word line stitch architecture of FIG. 1.
An alternative approach would be to eliminate the word line stitches and replace them with row decoders fabricated in place of the contacts. This would both increase the complexity of the memory device and increase the die size. An increase in the die size directly affects the cost of the device.
Accordingly, an alternative approach to incorporating the word line stitches, while adhering to the stringent physical restriction of the high-density memory devices is needed.